I'm wondering if I can set up my laptop as a host access point, but not using the normal wi[1] device. Instead using a WiFi enabled DSL router (ignoring the DSL functionality) connected to a second USB ethernet port.

First, a bit of history and level setting. Forgive me if this is all stuff you know.

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Bridges interconnect two (or more) Ethernets. Back when Ethernets were not all twisted-pair wiring and WiFi didn't yet exist, bridges interconnected multiple Ethernet networks, that were often different physical infrastructures: for example: coaxial cable (10Base2), fiber optics (10BaseF), and of course twisted-pair (10BaseT). A bridge is not a router, it just replicates any packets received on one media onto the other. This makes a bridge-interconnected series of Ethernets operate as if they are a single Ethernet. Bridges may include filtering capability.

These days, the most common bridge device available interconnects twisted-pair with Wifi. These are marketed as WiFi Access Points. They interconnect radio with wired Ethernet, and make the two Ethernets appear as if they are one.

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If your DSL router can be configured as a bridge, then yes, you can do this. If not, you will need to acquire a purpose-built WiFi Access Point that bridges to wired Ethernet.

You'd have to look through owners manuals or contact the vendors for guidance. But my guess is the answer is likely to be "no."

The Belkin specification you cited: RFC 1483, is for bridging IP traffic over ATM networks. I would guess that the D-Link's transparent bridge is for the same purpose. ATM is commonly used with ADSL networks under 622 Mb/s, according to Wikipedia. The bridging referred to here would be between an ISP's ATM-based DSL and a customer's Ethernet, and not used between two types of Ethernet.

Cheers, a USB card supported by the 'wi' device then looks like the only option, it would occasionally be useful to have a firewalled, etc., WiFi signal for mobiles devices and the likes that are not particularly security hardened.

It could be that you could operate one of these DSL devices as a router, rather than a bridge, (or as what used to be called "brouters") without having an upstream DSL connection. This would require the same level of investigation -- vendor documentation, vendor support staff -- with similar odds of success.

I've had ISP-provided gateway devices for home networks with built-in WiFi bridges -- all I've ever been able to do is provision the WiFi Ethernet segment (SSID, WEP/WPA, etc...) or disable the segment. APs I have managed have only ever been purpose-built bridging APs.

Even low-end consumer models of purpose-built APs will have better antennas and significantly improved functionality than any USB attached dongle, which are designed for client use, rather than attachment to servers or as APs. See http://daemonforums.org/showthread.php?t=9202 for more on the subject.

I take your point, I do though only plan to switch it on when I need to install an app. etc. and then bring the WiFi back down again.

It (namely the Belkin) did however quite happily function as a WiFi bridge and router on the modem only setting (a PXE boot worked fine). The only issue was one of the mobiles (a BlackBerry, while an Android worked OK) provisionally at least appeared not to search the network beyond the router itself for a DHCP server, it may be necessary to use the router built in server.

The next issue is how best to encrypt the Internet connection (includes devices that do not have proxy settings).

...a BlackBerry...appeared not to search the network beyond the router itself for a DHCP server.

DHCP discovery is a broadcast. You may find tcpdump(8) helpful to a diagnosis of the DHCP packets issued by and sent to that phone.

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...The next issue is how best to encrypt the Internet connection.

New topics require new threads at this forum. When you start your new thread, please describe the intended use-case and the network infrastructure for us, so we can make recommendations. These might include infrastructure solutions such as VPNs, or application-layer solutions such as TLS.